Activated carbon is widely used today as a filtration medium in industry and elsewhere for the removal of gaseous contaminants from gaseous and liquid streams where they constitute less than 1% of the fluid stream. The demand for this material is estimated at 220,000 metric tonnes per year and increasing at the rate of 5.4% per annum through 2002. This is due, in part, to increases in the output of chemical processes and more stringent environmental regulations worldwide. For instance, the environmental Protection Agency (EPA) has lowered emissions standards for several environmental pollutants. It should be noted that the emissions of volatile organic contaminants in the U.S.A. in 1998 totaled 1.62×107 Kg and solvent utilization accounted for 30% of these emissions.
In addition, the occupants of office buildings, the residents of private homes and institutions, the passengers in commercial aircrafts, trains and vehicles are increasingly concerned about the quality of the air they breathe. These concerns have become more acute with the implementation of energy conservation measures in these micro environments, and the increased usage of outgasing synthetic materials. This has led to an increased interest in ventilation systems capable of controlling the presence of gaseous contaminants in breathing air. Such systems invariably make use of activated carbon to control these pollutants.
Most of the activated carbon used today for the removal of gaseous contaminants from air streams at concentrations <1% is either granulated or pelletized activated carbon or powdered activated carbon (PAC) usually placed in trays. The contaminated air stream is routed through a bed of activated carbon which adsorbs the gaseous contaminants. The purified air stream is either recycled or discharged to the atmosphere. Inherent problems associated with such systems include high pressure drops and the periodic replacement of the spent carbon, a labor intensive, potentially hazardous and costly procedure. Alternatively, carbon can be incorporated in a matrix or bonded to fiber and shaped as panels, blocks or slabs as described in (WO 94/03270). Although this addresses the problem of high pressure drop across the filtration medium, it leads to a decrease in adsorptive capacity of the bonded carbon and the need for periodic replacement remains.
Activated carbon is also available as activated carbon cloth both woven or knitted, and as a felt. It can be used to make very thin carbon beds having very low pressure drops and with an adsorptive capacity equivalent to deeper granular carbon beds. It is ideally suited to air purification. However, as for the other forms, it must periodically be replaced, and the time between replacements can be comparatively short.
The spent carbon, in all its forms, is either regenerated or replaced when its effectiveness falls below an acceptable value. Replacement with virgin activated carbon is costly and laborious. Movement of the spent carbon off site for regeneration incurs transportation and labor costs and degradation of the medium. The process of regeneration of the adsorptive capacity of the spent carbon invariably involves the heating of the carbon beds. This heat is usually supplied externally by the use of hot air or steam, or by placing hot elements in the carbon medium (U.S. Pat. No. 5,187,131). The regeneration gases burn the carbon medium with a concomitant loss in both the amount of active carbon and the adsorptive capacity. Similar procedures are available for on-site regeneration of spent carbon.
Regeneration of spent carbon by means of vacuum procedures have been largely ineffective to date, because adsorbate volatilization requires thermal energy and vacuum desorption has a chilling effect on the carbon which cannot be offset because the vacuum environment is known to be a very efficient thermal isolator. Hence, for all available processes to date, heat energy cannot be added to the chilled carbon during vacuum adsorption. At best, the process requires an inordinate amount of time to achieve acceptable regeneration.
In general, for all the aforementioned regeneration procedures, the regenerated activated carbon never attains its original adsorptive capacity because there is a residual adsorbate which resists removal. This is especially common with “in-situ” steam regenerated activated carbon. The result is that eventually the regenerated carbon does not satisfy contaminant removal requirements, and must be replaced.
It is known in the art that activated carbon is capable of conducting electricity. The resistance properties of this material are such that useful heat can be generated in this manner (U.S. Pat. No. 6,107,612). Attempts have been made to generate the heat required for regeneration of spent carbon by making use of this property of carbon, (DE 4104513). However, only limited successes have been achieved with this procedure when applied to granulated/pelletized/powdered carbon, because of non-uniform heating patterns, hot spots and short circuits.
Better results have been obtained by passing an electric current through the carbon cloth thus generating the heat required for desorption within the sorbent medium itself where the thermodynamics of the adsorption and desorption processes apply (U.S. Pat. No. 5,912,423) This method is inherently more thermally efficient than prior art methods. However, the method requires that a purge stream of air or inert gaseous be used to convey the desorbed contaminants away from the cloth. The use of air is not recommended since it invariably leads to significant loss of carbon over time by oxidation; and the method requires the use of large volumes of inert gaseous during the regeneration phase which may take three hours or more depending on loading and adsorbate characteristics. This method also requires that technically sophisticated means, usually cryogenic, be used to remove the contaminants from the large volume of inert gases used.